For my cat curiosity trumps any kind of fear. He is scared of many things, anything with a loud noise will set him off, but he just gets a bit of distance between himself and the "danger" and then starts to investigate. At the beginning he was much more jumpy, now even the vacuum cleaner can only push him to move a bit.
Strangely his biggest scare was with a remote controlled car. It wasn't loud or anything, it just moved slowly, and I thought it would be nice to make him play with that. But instead as soon as that little car moved he jumped and assumed the arched back position. Perhaps it's because he had first seen it as a static object, and had already let his defenses down when the car moved.
One of my sister's cats, instead, is really easy to scare, everything sets him off! Once he made a glass vase fall and he ran away. She couldn't find it for hours and she called me on the verge of tears. He was all squeezed up in an interstice between the fridge and the wall behind it, and the way he looked up with big wide eyes when we found him was really priceless!
